Online music sales have struck a sour note. Sales of compact discs and tapes over the Web have dropped 25% this year, says a study to be released Monday. And the trend shows no sign of slowing. Through the first nine months of the year, consumers spent $545 million to purchase music on the Web vs. $730 million a year ago, says comScore Media Metrix, a unit of researcher comScore Networks Inc. The drop in online music sales runs counter to overall product sales on the Web. They're up 35%, says comScore.

The decline in music sales is puzzling and disturbing, says Peter Daboll, president of comScore Media Metrix. "When you see online sales in total growing pretty consistently at 30% growth year over year, to see one segment falling off is pretty surprising," he said.

The growth of file-sharing services are helping slow online music sales, says Daboll. Such services let consumers get songs over the Internet for free. "Consumers have a choice between clicking for purchase or getting something at a file-sharing site for free," he said.

Just a few years ago, sales of CDs over the Web looked like a sure thing. Consumers flocked to online CD stores because they could find thousands of artists and titles on one site - far more than a corner record store could hold. The CDs arrived a few days later in the mail.